Electrostatic microphone circuits



Jan. 5, 1960 R, MORGAN 2,920,140

ELECTROSTATIC MICROPHONE CIRCUITS Filed March 14, 1958 4 Sheets-Sheet 1 a /6 F 4! zz i l i- 2 17 f 1/ 50 46 1 j] x IN V EN TOR.

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ELECTROSTATIC MICROPHONE CIRCUITS Filed March 14, 1958 4 Sheets-Sheet 3 l/NE 112 EE T T T 1;; Al/cWOF/IME :5;

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BYZ 2 ATTOIPIVEL Jan. 5, 1960 A. R. MORGAN 2,920,140

ELECTROSTATIC MICROPHONE CIRCUITS Filed March 14, 1958 4 Sheets-Sheet 4 Ey' .]d

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2,920,140 ELECTROSTATIC MICRORHQNE I GI Adolph R. Morgan, Princeton, N.J., assignor to Radio (lorporation ,of America, a corporation of New York Applisafion March 1 a, S i o- 1 .8 Glaims. .(Cl. 17911) This invention relates to electrostatic microphones, and more particularly to circuits for operating such micro- .phones.

In a typical electrostatic microphone, there are in- .cluded a pair of plates, at least one of which is a vibratile .element. A .polarizing voltage is established across .these plates. The voltage output of the microphone results from changes in this-polarizing voltage due to the vibration of the spacing between the two plates. Before an appreciable output .voltage can be obtained from the microphone, a polarizing voltage of suflicient magnitude must be established between the plates thereof. To provide this polarizing voltage, the capacitor formed between the electrostatictransducer is charged. With previous circuitry for electrostatic microphones and transducers, anappreciable charging time is required before 'a polarizing voltage 1 of operating magnitude becomes established. -In certainconventional circuits for operating electrostatic microphones, the .Lelementsof the transducer arecharged -by the grid current flowing'from a. grid element of anelec- -tron:tube. ,Since the .grid current of an electron tube-is ordinarily small under operating conditions prior electrostatic microphones were not immediately-available for use as .soon as .thecircuitry therefore vwas energized. i fInaccordance with the present invention, an improved circuit for operating an electrostatic transducer,.as may 'be incorporated in'a microphone, is provided including .meansrforprovidinga direct connectionbetweenla source of .voltage for-establishing the operatingpolan'zing voltage in the microphone in combination withanamplifier. The circuit also has the feature-of increasingthe sensitivity of the microphoneby providing for the establishrment ofpolarizing voltage of largermagnitude thanhere- .toforepossible. i Y

'It is therefore an object of the, presentinvention to provide improved circuits for operating electrostatic transducers. i

It is a further-object of the present invention to provide circuits for operating an electrostatic transducer which .are capable of developing-polarizing voltage-of greater magnitude between the elements-of the microphone than with-circuits heretofore available.

' It is a still further object ofthe present-inventionto provide circuits for operating electrostatic microphones which decrease the timerequired for establishment of tlie polarizing voltage in the electrostatic transducer thereof.

Other objects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to-which the invention is directed from a reading ofthefollowing description in combination with the accompanying drawing in which:

Figure l is a sectional view of a directional electrostatic microphone;

' :Figure '2 is a sectional-view.- of the microphone shown in Figured, the section being taken along the line 2-2 of Figure 1.;Whcnyiewed in th zdirection of the arrows,

Figurcv 3 ,isanother sectional view. .of the microphone United States Patentin 7 2,920,140 Ratented Jan. 5, 1960 ice shown in Figure 1, the section being taken along the line 3-:3 of Figure 1 when viewed in the direction of the arr ws,

Figure 4 is still another sectional view of the microphone shown in Figure l, the section being taken along the line of Figure 1 when viewed in the direction of the arrows.

Figure 5 is a side view of a protective case for the microphone shown in Figure 1, the case being partially broken away to show the microphone,

Figure 6 is a schematic diagram of the acoustical network of ,the microphone shown in Figures 1 to 5,

Figure -7 is a simplified acoustical network of the microphone shown in Figures 1 to 5,

Figure 8 is a fragmentary front View, partially in section, ,of a modification of the directional electrostatic microphone shown in Figures 1 to 5,

Figure 9 is a schematic diagram of a circuit which may be used with the microphone shown in Figures 1 to 5,

Figure 10 is a curve showing a typical grid current characteristic for van electron tube,

Figure 11 is a schematic diagram of a circuit in accordance with an embodiment of the present invention which may be used with the microphone illustrated in Figure 12 is a schematic diagram of a circuit in accordance with another embodiment of, the present invention which :may be used with the microphone illustrated in Fig re Figure '13 is a schematic diagram of another circuit in accordance withstill another embodiment of the present invention which may he used with the microphone illustrated in Figure 8, and

Figure 14 is a schematic diagram of stillanothercircuit in accordance with a further embodiment ofthe present invention which may be used with the microphone illust atedin i e The structure and construction of a directional electrostatic microphone having the features provided by the present invention are shown in Figures 1 to 4. The micro- .phone isenclosed in a case 10 of light-Weight metal such as aluminum. The case includes two separable cylindrical parts 11 and 13 which-are joined to each other, as at a joint 14. The bottom one ,13 of the case parts is closed at its lower end. This closed end is provided with a centrally located hole. A cylindrical, internally threaded boss 12 projects from the bottom of the case. Electrical connections to the elements of the microphone may .be brought through the boss 12 and the hole in the bottom ofthe case. The major portion of the elements of the electrostatic transducer are located in the top part 11 of the case 10. The bottom of the case may contain components of an electrical circuit for operating the transducer. In the drawing, asmall electron tube 24 is shown as being located within the case.

The electrostatic transducer includes a vibratile diaphragm .16. This diaphragm may be a disc of flexible A polyester plastic material, such as Mylar (a trademark of the Dupont Company) may be used. The diaphragm 16 shown in Figure 1 of the drawings is a disc of Mylar material which is coated with a conductive material on at least one surface thereof. In one embodiment, the surface which is coated faces outward of the case 10 so that it.may cooperate with the other element of the electrostatic transducer. Aconductive material, such as gold may be coated onto the surface of the Mylar sheet by any suitable technique, such as sputtering. The diaphragm is secured to the top of the casing lt). by means of a clamping ring 18. This clamping ring, may ,also be made of some conductive material such .as aluminum. Aplurality ofscrews 20, which are spaced at equal distances from each other around the ring 18 are used to fasten the ring and diaphragm 16 to the top of the case 10.

An electrode 22 cooperates with the diaphragm 16. This electrode is illustrated as a disc shaped plate of conductive material, such as brass. Another term descriptive of the electrode 22 is back-plate. The top surface of the back-plate electrode 22 is spaced closely adjacent to the rear surface of the coated diaphragm 16. A plurality of holes extend through the electrode 22. A large hole is located at the center of the electrode 22 and is a blind hole which extends from the back surface of the disc. The holes are arranged along circular paths at successively greater radial distances from the center of the disc. The ones of the holes which are along the path at next to the largest radial distance from the center of the disc are blind holes 26 and extend only a predetermined distances into the disc from its front surface. The arrangement of holes, and particularly the blind holes 26 provides desirable acoustical characteristics for the microphone. The edge of the electrode 22 at the front surface thereof is slight beveled. This slight bevel also improves the acoustical characteristics of the microphone.

The electrode 22 is supported by a bushing 28 of insualting material, such as nylon. This bushing 28 also supports most of the other acoustical elements of the microphone. An acoustical resistance element provided by a disc 30 of porous material is disposed directly against the rear surface of the electrode 22. This acoustical impedance element is selected to provide an impedance characteristic which is substantially resistive. An element of controlled porosity has been found suitable. For example, the material of the disc 30 may be filter paper of a selected grade. Ordinary chemical laboratory quality filter paper may be found suitable.

It will be noted that the bushing 28 is provided with a plurality of apertures 32 in the Walls thereof. The

axes of each of these apertures is in the same plane.

This plane is located at a predetermined distance behind the diaphragm 16, the electrode 22, and the acoustical resistance element provided by the filter paper disc 30. The apertures 32 in the bushing 28 are exactly aligned with apertures 34 in the walls of the casing 10. Sound waves may be conducted to the rear surface of the diaphragm through the apertures 32 and 34, the resistance element 30, and the back-plate electrode 22. Sound waves may be conducted through the apertures 32 and 34 into the cavity defined at the rear of the case by way of other acoustical elements 36, 38, and 40, and the element 50. These elements permit sound pressure to be developed at the rear of the diaphragm which is related to the sound pressure developed at the front surface of the diaphragm in a predetermined manner. The relationship between the sound pressures is such that the desirable directional characteristics for the electrostatic microphone are obtained. The apertures 32 and 34 together with the filter paper disc 30, the apertured electrode 22 and the acoustical elements 36, 38, 40, and 50, which communicate the apertures 32 and 34 with the cavity at the rear of the case 10 comprise means for shifting the phase of the sound pressure developed at the rear surface of the diaphragm 16 with respect to the sound pressure at the front surface of the diaphragm. As will be more fully set forth hereinafter, certain of these elements present an acoustical network having a substantially resistive impedance characteristic.

The other acoustical resistance elements, which may be provided by discs 36, 38, and 4d, of filter paper, similar to the disc 39 of filter paper used for the element at the rear of the electrode 22, are disposed behind the apertures 32 and 34 to provide an acoustical network which is substantially resistive. The acoustical resistance elements provided by discs 36 and 36 are spaced from each other by means of other discs 42 and 44 of foam rubber. The acoustical resistance element discs 36 and 38 are spaced from each other by further discs 46 and 48 of foam rubber material. The foam rubber discs are substantially equal in diameter to the inside diameter of the bushing 28. A circular plate 50 of insulating material, such as nylon, is disposed between discs 38 and 40 of a filter paper. This circular plate 50 is ported with a plurality of apertures. A centrally located hole extends through all of the discs of filter paper 30, 36, 38 and 40, the discs of foam rubber 42, 44, 46, and 48, and the apertured plate 50. A bolt 52 of conductive material extends through the plate and the disc element and this screws into the centrally disposed blind hole of the electrode 22. There is thereby provided a unitary structure of relatively inexpensive, easily manufactured and assembled elements which provide the desired acoustical elements of the microphone. The entire structure may be readily inserted into the top portion of the case 10 and fastened into place by means of the set screws 54.

The foam rubber discs, 42, 44, 46, and 48, provide the requisite, predetermined spacing between the acoustic held resistance elements provided by the filter paper discs 30, 36, 38, and 40. Any material, such as some plastics, having equivalent acoustical and mechanical properties, may be used in place of foam rubber in constructing the discs 42, 44, 46, and 48. The discs 42, 44, 46, and 48, have an acoustical impedance substantially less than the acoustical impedance provided by the filter paper discs. Therefore, the discs 42, 44, 46, and 48,

' have a negligible effect upon the acoustical characteristics of the microphone. However, the discs insure proper assembly of the other elements of the microphone in a convenient, inexpensive and efficient manner.

The electron tube 24 is mounted on the closed end of a hollow cylindrical member 56 which is closed at only one end. This member 56 is made of insulating material, such as nylon, since it functions as a socket for the tube 24. The cylindrical member is disposed in the bottom part 13 of the casing 10 and rests upon the bottom casing. A post 58 of insulating material such as nylon is inserted into the end of the cylindrical member 56. A lead 60 is held by the post. This lead 60 may be the grid lead for the tube 24. Thus, the lead 60 is held securely and vibration of the sensitive elements of the tube is prevented which might otherwise cause microphonics. The other leads 62 are inserted through socket members 64 in the bottom of the cylindrical member 56. Conductors 66 in a cable 68 are connected to the socket members 64. The cable 68 is mounted in a plug 70 which is screwed into the boss 12 on the bottom of the case 10. The conductors 66 in the cable 68 are connected to other components of the circuit for the microphone. These elements will be discussed later.

Referring now to Figure 5, the case 10 of the microphone is shown suspcnded within a protective outer 72. This casing is constructed of thin sheet metal and is perforated. The inside of the casing is covered with a porous cloth screen 74. By reason of the perforated casing 72 and the cloth screen 74, the microphone is protected against wind noise of the type encountered in the motion of microphone through the air on a boom. The boom support 76 is connected to the casing 72.

The microphone is supported within the perforated casing 74 by means of a yoke structure 78. This yoke structure includes a circular strap 80 having bifurcated elements 82 extending from opposite edges thereof. The bifurcated elements 82 are connected to the inside of the casing by means of yieldable supports, such as rubber bands 84. The microphone is therefore protected against external vibration. The cable 68 connects to the microphone element.

In operation, the microphone will be positioned with its cylindrical axis in the direction of the sources of sound which are to be picked up. For example, the microphone may be turned so that its cylindrical axis points -light weights and directivity :of the uniaxial electroopposite surfaces of-the diaphragm '16, In other words, "the microphone is a sound pressure gradient responsive microphone.

. actuating sound :waves.

.5 toward the performing actor on the sound stage withthe end of the microphone having the diaphragm facing the actor; Unwanted noise is likely to be produeeddn a random manner due to production equipment around the sound stage. This noise will arise from sources all around the microphone and particularly at the back thereof. The desired sound will first strike the front surface of the diaphragm 16. After a phase :and'time delay due to the displacement of the apertures 34 and 32 behind the diaphragm 16 and the acoustical network in the microphone, the sound waves will -reach the rear surface .of the diaphragm 16. For sounds along the cylindrical axis of the microphone and emanating from the front of the diaphragm 16, sound pressures will be produced at the front and rear surfaces of the diaphragm 16 having a 7 maximum phase displacement with respect to eachother.

Thus, significant forces will be applied to the diaphragm 16 and the diaphragmwill vibrate at relatively large amplitudes for sounds coming from the front of the microphone and along its cylindrical axis. Unwanted sounds and noise coming from the rear of the microphone will be subjected to a phase and time delay in passing through the apertures and acoustical network at the rear of the diaphragm. A similar phase and time delay will occur before such unwanted sounds reach the front of the diaphragm. Therefore, the sound pressures established at the front .and rear surfaces of the diaphragm for unwanted sounds will be substantially in phase. Consequently, the diaphragm will not vibrate due to such unwanted sounds :or willvibrate at a considerably smaller amplitude than for sounds which are to be picked up. The maximum sensitivity of the microphone will therefore be forsounds from the front of the microphone which are directed along .the cylindrical axis thereof. Because of the symmetrical disposition of the apertures, the microphone will be uniformly unresponsive to sounds from all sides which are incident upon the diaphragm due to sources at the rear of the microphone. The microphone .will be uniformly sensitive to sound from sources .at the front thereof. The microphone may therefore be considered .to have .a uniangular response characteristic. Because the axis of maximum sensitivity of the microphone corresponds to the axis of the microphone assembly, the directional, electrostatic microphone provided by the present invention may be termed a uniaxial microphone. Uniaxial microphones of the electromagnetic type, which may have ribbon vibratilc eleme m, are available. Such microphones are illustrated in Olson et al., Patent No. 2,680,787, issued June '8, 1.954. The uniaxial electrostatic microphone provided by the present invention is therefore directly .compatible with sound systems which may be uniaxial electromagnetic microphones, and will beespecially useful where the features of small-sized,

static microphones are desired.

It follows from the above discussion that the microphone provided in accordance with the present invention is responsive to the difference in sound pressure on the The pressure gradient is established by the phase shift introduced in the apertures 32, 34 and phase shift means, including the acoustical elements behind the diaphragm. Since phase shift imposed on acoustical waves by such phase shift means varies in accordance with'the frequency of such waves, it would seem that the pressure gradient, which is the actuating sound-pressure for the microphone, wouldbe related to the frequency. It is desirable-that'the response of the microphone be independent of the frequency of the Expressedin other words, it is desirable that there be a constant relationship between e actuating sound pressure and the open circuit voltage output 'of -the microphone.

"in -sound, complex; 'acoustioalnetworks were combined with the vibratii g elements of the microphone to provide the requisite uniform response with respect to the frequency'of the actuating sound waves. Another alternative additional circuitry. The former increases the cost of the microphone and makes repair difiicult. The latter requires careful and special design of the amplifiers associated with the micropho e. Thus, prior directional electrostatic microphones may not "be compatible with the ava lab e amplifier n he s nd s d o In, ac e ith the P ese n ent on th eq te c n relationship be en he. ol a ou p f h m ophone n he a ati g ou d p es u e nd a. u o m response with respect to'frequency is obtained with the ou l ne pre iti "by t e im e ac usti a elem nts, s c a e fi er paper di s 3 6, a 0, which a x n i e tobui d'and y to a nta These acoustical elements provide an acoustical network which is resistance controlled, in acoustic sense. The

where e=open circuit output voltage of the transducer, in volts: e =polarizing voltage or potential gradient between the elements, in volts: i a=s pacing between the surfaces 'ofgthe elements, in centimeters; and x =amplitude of vibration of the movable element,

' which is the diaphragm -16, incentimeters.

It appears from Equation '1 that there is a constant relationship between the amplitude of vibration of the diaphragm 16 and the open circuit output voltage of the transducer. In accordance with the present invention,

the actuating sound pressures in the microphone are controlled so thatforces for actuating the diaphragm are related to the sound pressure in ,a manner to establish .a constant relationship'between the actuating sound pres- .sures and the open circuit output voltage of the microphone, notw ithstandingthat the microphone is a gradient microphone and the intensity of the actuating sound pressures is proportional tofrequency, Thus, the microphone will have a uniform response with respect to the frequency of the actuating sound waves.

The actuating force on the diaphragm may be expressed as follows: i 1

fMD=j 1P where,

f =actuating force, in dynes; K =constant of the vibrating system;

p=sound pressure, in dynes per square centimeter;

f=frequency ofthe actuating .soundwaves, in cycles per second; and A: area of the diaphragm, in square centimeters.

The amplitude of vibration of the diaphragm 16 is given by:

fun ii zu Equation 4 shows that the amplitude of vibration of the diaphragm will be independent of the frequency of the actuating sound waves, if the mechanical impedance characteristic z is not dependent on frequency and therefore is a mechanical resistance. Thus, a resistance controlled vibrating system, as is provided by the acoustical elements disposed behind the diaphragm 16 in the microphone illustrated in connection with Figures 1 to 4, will have an output voltage Which has a constant relationship with the actuating sound pressure. Consequently, the response of the microphone will be uniform with respect to frequency.

The resistance controlled nature of the electrostatic uniangular microphone shown in Figures 1 to 4 may be illustrated in connection with the schematic diagrams of the acoustical network thereof which are shown in Figure 6.

In that network:

12 is the sound pressure at the diaphragm 16;

p is the sound pressure at the apertures 34, 32;

M is the mass of the diaphragm 16, and the air load on the front of the diaphragm;

C is the acoustical capacitance of the diaphragm;

r is the acoustical resistance behind the diaphragm;

M is the inertance of the apertures 32, 34;

r is the acoustical resistance of the apertures;

r is the acoustical resistance of the termination; which includes the elements behind the aperture; and

C is the acoustical capacitance of the volume of air in the case 10. 7

In order to provide the uniform response and directivity of the microphone with respect to frequency, the acoustical resistance r of the first branch 86 is the controlling acoustical impedance in that branch 86. For the same reason, the acoustical resistance r in the parallel branch 88 is the controlling acoustical resistance in that branch. Under these conditions, the acoustical network can be simplified by eliminating the acoustical elements which have a negligible effect on the system. The simplified acoustical network is shown in Figure 7. This acoustical network may be referred to in analyzing the operation of the vibrating system of the electrostatic uniangular microphone provided by the present invention. Thus, the volume current in the branch 86 due to the pressure is given in equation:

where,

X =volume current, in cubic centimeters per second;

P =actuating pressure at the diaphragm of the micro phone, in dynes perv squarecentimeter;

r =acoustical resistance behind the diaphragm, in acoustical ohms;

r =acoustical resistance of the termination, in acoustical ohms; and

M =the inertance of the apertures, in grams per (centimeter)*.

The volume current in the branch 86 due to the pressure p is given by:

where,

X =the volume current, in cubic centimeters per second;

and I P =the actuating pressure at the aperturesof the microphone, in dynes per square, centimeter. In order to determine the characteristics of the actuating sound pressures on the microphone, it will be assumed that the reference point of Zero phase is at the front surface of the diaphragm 16 of the microphone. Then, the expression for p may be Written:

l 1=l o1 i (7) where, p =the amplitude of p in dynes per square centimeter,

and z time, in seconds.

The expression for p is written:

P2=p026j(wt+ cos 0) (8) where, Po2=amplitude of p in dynes per square centimeter; d=effective acoustic path from the diaphragm to the aperture, in centimeters; k wavelength of the actuating sound wave, in centimeters; and 0=angle between the axis of the microphone and the direction of the incident sound wave.

The resultant volume current in branch 86, which is the voltune current of the diaphragm 16, is given by:

of Acoustical Engineering, by H. F. Olson, 2nd ed., pp.

264-65, the'volume displacement of the diaphragm, 16 can be expressed as Where,

X =volume displacement of the diaphragm in cubic centimeters; and

K =constant of the acoustical and electrical systems.

The amplitude of the diaphragm is given by:

Combining Equations 10 and 11, the open circuit output voltage developed by the microphone is given by:

where, K =the sensitivity constant of the acoustical and electrical systems.

It will be observed that the directivity pattern as given by Equation 12 is a cardioid. This directivity is improved, in the microphone shown in the drawings, by location of the apertures 32, 34 along the cylindrical body of the case 19. The improvement is provided by diffraction phase effects which are a function of the angle of the assuage An examination of Equation 13 shows thatthe directivity of the microphone. is greater than that of: a simple cardioid microphone. For example, atninety degrees with respect to the axis of the microphone, the response is down eight decibels from that at zero degreesor on the of the microphone as compared to six decibels for the cardioid pattern.

The frequency response of the microphone may be improved in the high frequency region by means of the blind holes 16 which are arrayed in the back-plate electrode 22. The presence of the blind holes in the closed area at the back surface of the diaphragm 16 establishes an auxiliary acoustical network which has a negligible effect uponthe operation of the electrostatic transducer and its vibrating system except in the high frequency region of the response characteristic of the microphone. In that region, the auxiliaify network produces a resonance effect which improves the sensitivity of the microphone. Thus, the disclosed improved uniangular directional microphone has a uniform response with frequency for all audio frequencies aiid is compatible with high fidelity sound and recording sys e The microphone illustrated in connection with Figures l to 4 may be used with conventional circuitry to'derive the output voltage therefrom. One such circuitry is shown in Figure 9 of the drawings. In Figure 9, the microphone is schematically illustrated 'as comprising the casing 10, the diaphragm 16 and the electrode 22. The electrode 22 is positioned in back of the diaphragm 16 and may be referred to as the back-plate electrode of the electrostatic transducer. The casing 10. of the microphone is connected to a point of reference potential, such as ground. The diaphragm 16 which is coated with a conductive material is also connected to ground since it is in contact with the casing '10. The physical connection of the diaphragm to the casing 10 is shown in greater detail in Figure l. The circuit shown in Figure 9 includes the vacuum tube 24, having a plate 90, grid 92, a cathode 94 and a heater or filament 96. The filament tnay be connected to a source of heating voltage, which is desirably a direct current source. The grid 92 is connected to the back-plate electrode 22. The plate 90 is connected toa source of operating voltage shown on the "drawing 'at B+. I The cathode 94 is connected to ground throughIa cathode resistor 98 and to the primary winding .of an output transformer 100 through a coupling capacitor 5102. It will be observed that the grid connectioh to the back-plate '22 may be 'by way of the lead '60 which is shown in Figure l. I

As the diaphragm lti vibrates, the distance between the adjacent surfaces of the diaphragm 16 and the backplate electrode 22 varies in accordance with the actuating sound pressure. Thisvariation causes the voltage between the plates of the capacitor constituted 'by the diaphragm 16 and to correspondingly vary as the capacity changes. This varying volt-age appears at the grid 92.

"The tube circuit functions as a cathode follower to p'r'ofitie output voltage across the cathode resistor 98. fIli'e direct current voltage across the cathode resistor '98 is blocked by the coupling capacitor 103 so that'o'nly al- I 'ter'n'ating current passes through the primary winding of the transformer 100. The useful alternating current "o'utpiit signal of the microphone may be derived across the secondary winding of the transformer. 100.

The potential gradient across the electrostatic tran'sfducer from the back-plate electrode 22 to the diaphragm "16 is established when the operating potential is applied to the tube'24. The-electrostatic transducer functions as a capacitive element in the circuit, and charges to establish the polarizing voltage thereacro'ss. However, the "tiiheietiuired for the capacitor -to charge to its operating fvo'ltag'e 'may be appreciable. As may be ob's'ervedfrom =Eq'uati'o'n ll, the output "voltageof the electrostatic trans- :du'eei is proportional to the potential *to which the elee 'theretit. are'charge'd. Until this potential is with :s'hoick -to'-ithe user of the microphone.

1O .ciently great, the, voltage output of the transducer and the signal output of the circuit is not of sufficient'magnitude to properly operate amplifiers which are associated therewith.

With the circuit of Figure 9, the amount of time required to charge the electrostatic transducer may be appreciable. An analysis of the grid current-grid voltage characteristics of the vacuum tube 24, such as shown in Figure 10, will determine the reason for the charging time. A free grid, i.e., a grid with no external connection, must assume the condition of zero grid current. Such a point is indicated as, A, in Figure 10. The free grid-voltage for the majority of electron tubes is negative with respect to the cathode and is small and only a few volts in magnitude. The grid 92, in Figure 9, does not have a conductive return path connected thereto. sense and must ultimately assume the zero grid current condition, A in Figure 10. With a small negative grid voltage, appreciable current flow takes place through the tube. 24 from plate to cathode. A large voltage drop occurs across the cathode resistor 98. This voltage drop raises the cathode substantially positive in voltage with respect to ground. Since the grid 92 and the transducer element 22 will have assumed a small negative voltage with respect to the cathode 94, the transducer element 22 will be charged to approximately the same voltage as the cathode 94,

r In 'the interval preceding activation of the circuit of Figure 9 by applying B+ to the plate 90, all elements of the circuit will be at zero or ground potential. At the instant of applying 13+ to the plate the transducer element '22 and the grid 92 must, by the nature of the transducer capacitance, remain at substantially ground. potential. The plate current of the vacuum tube 24 will therefore, in that instant, increase to a value which will raise the cathode 94 to a voltage approximating the grid cut o'lf potential of the vacuum tube. In the first instant of activation of the circuit of Figure 9, the grid 94 will, therefore, have a voltage several volts negative with respect to cathode 94. This negative grid voltage will be at Ya point such as B in the grid current curve deipicte'd in Figure 10. The resulting grid current is quite small under these conditions. In the interval following the instant of activation of the circuit of Figure 9, the electrostatic transducer capacitance is charged by the flow of grid current from grid 94 to the back plate electrode 22. Since the grid current, as described above, is "quite small the charging time of the transducer capacitance must be appreciable.

Improved circuits decreasing the charging time may be provided. Illustrative embodiments of such circuits are shown in Figures 11, 12, and '13, of the drawings. The circuits of Figures .12 and 13 have the additional feature of causing the voltage across the elements 16 and 22 ofthe electrostatic transducer to increase to an extent thattheyfare larger than the polarizing voltages available with the circuit shown in Figure 9. It will be noticed, however, that with the circuits shown in Figures 1 1, 12, and 13, the diaphragm 16 of the microphone is not .con-

nected .to ground. Instead, it is connected to the source of operating potential.

In the embodiment of the microphone shown in Figure 8, means are provided .for insulating the diaphragm 16 so that it may be connected to a source of high voltage andat the same timeeliminate "any hazard of electrical The parts of the embodiment of the microphone-shown in Figure '8 which are rsiniilarto the corresponding parts of the microphone shown in Figures l to 4:are designated with like referenc'e numerals. It will be appreciated, however, that the Therefore, it is a free grid in the direct current 11 uniangular microphone similar to the microphone shown and described in connection with'Figures l to 4. This microphone includes a case having an electrostatic transducer in the upper part thereof and electrical components, such as an electron tube 102 in the lower part thereof. The electrostatic transducer includes a diaphragm 16 coated on at least one surface with a conductive material. A back plate electrode 22 is spaced adjacent the uncoated surface of the diaphragm 16.

and establishing the directional response characteristic,

of the microphone. The acoustical operation of this microphone was discussed above.

The diaphragm 16 is supported on top of the case 10 in a manner to be insulated, from the case. A ring 104 of'insulating material which may be an insulating plastic, such as Bakelite, is disposed at the top of the case. The diaphragm rests on this ring. Another ring 106 of conductive material such as copper is placed on top of the diaphragm and overlies the plastic ring 104. Another ring of insulating material, such as a plastic material, is placed on top of the conductive ring. Thus, the edge of the diaphragm 16 and the contact ring 106 are sandwiched between the plastic rings and are insulated by means of these rings from the case. A plurality of screws 10 extend through holes in the rings 104, 106, and 108,

and in the diaphragm. The diameter of the holes is greater than the diameter of the screws so that the screws are not in contact with the conductive ring 106. These screws are attached to the material at the top of the case 10. The screws 110 may be equally spaced from each other around the rings as was the case with the screws 20 shown in Figures 1 and 2.

A tab 112 of conductive material is attached to the conductive ring 106. A lead 114 is connected to this tab by means of soldering. A terminal cover 116 of insulating material, which may be cemented to the plastic rings 104 and 108 encloses the soldered junction. A screen 118 of stiffened cloth may be used to cover the entire top portion of the microphone. This screen 118 may be secured to the top of the microphone by means of the screws 110. Thus, the entire unit is electrically insulated and isolated so as to eliminate any shock hazard.

The lead 114 may be connected to the cable at the bottom of the microphone case. A plug connection similar to that shown in Figure 1 may be used. This lead 114, as will be observed from the circuit diagrams of Figures 11 to 13, is connected to the source of operating potential for the microphone.

In Figure 11, a microphone of the type shown in Figure 8 is schematically illustrated as including the diaphragm 16, the back plate electrode 22 and the case 10. The lead 114 is connected to the diaphragm and is in sulated from the case as in the diaphragm. The circuit of Figure 11 includes a power supply 120 for providing low voltage direct current for operating the filament of the electron tube 102. The power supply 120 also provides higher voltage direct current for operating the circuit and the microphone. The power supply transformer 128 has a primary winding which may be connected to the power lines and low voltage and high voltage secondary windings 130 and 132, respectively. The low voltage alternating current across the low voltage winding 130 is rectified by means of a pair of diodes 134. The rectified alternating current from the diodes is filtered is a resistance-capacitance filter circuit 136. The filament 138 of the tube 102 is connected across the output of the filter 136. Thus, the tube 102 is heated by direct current so as to prevent the introduction of hum into the circuit.

erates into a filter network 150. The filter network 150 is a multi-stage filter of the resistance-capacitance, type so that a substantially pure direct current voltage output is obtained. The high voltage terminal of the high voltage supply is labeled l-B and the low voltage terminal of the power supply is labeled B. The high voltage terminal of the power supply is connected to the plate 152 of the tube 102 and simultaneously to the diaphragm 1 6 of the electrostatic transducer in the microphone. The low voltage terminal indicated at B is connected to the cathode 154 of thetube 102 through a cathode resistor 156. The signal voltage output of the circuit is obtained across the cathode resistor through a blocking capacitor 158 and an output transformer 160.

In operation, the electrostatic transducer is immediately charged upon energization of the circuit, as by connecting the transformer 128 primary winding to the power line. A high polarizing voltage is established between the diaphragm 16 and the back plate electrode 22. When the high voltage from the power supply, indicated at B+ is applied to the diaphragm 16, the capacitive electrostatic transducer initially acts as a short circuit and applies a positive voltage to the grid 162 of tube 102. As may be seen in Figure 10, a positive grid causes a large amount of grid current to flow and the transducer ele- High voltage direct current is provided by a half wave ment immediately charges to a suificiently high polarizing voltage to permit proper operation of the electrostatic microphone. The plate current, through the tube, is initially higher than'normal, since the grid 162 is positive. However, the bias voltage developed across the cathode resistor 156 is not sufiicient to cut off conduction through the tube before the capacitive transducer element is charged to requisite polarizing voltage by grid current conduction.

When the circuit reaches its quiescent operating condition, the grid voltage will return to a magnitude such that grid current conduction is zero. The operation takes place because of the characteristics of the grid 162 as a free grid in the circuit as explained above.

After initial energization of the circuit of Figure 11, the polarizing voltage across the transducer elements 16 and 22 will be approximately equal to the voltage drop across the plate to cathode of the electron tube 102. The free grid potential is small compared to the potential of the cathode 154. More accurately, the polarizing voltage will be equal to the supply voltage at B+, added to the voltage from the grid 162 to the cathode 154, less the voltage drop across the cathode resistor. The last voltage will be substantial in magnitude, consequently the polarizing voltage will be correspondingly less than the supply voltage.

In the circuit of Figure 9, the electrostatic transducer eventually becomes charged to a polarizing voltage equal to the voltage drop across the cathode resistor 98, less the bias voltage from the grid to the cathode. In other words, the polarizing voltage in the circuit of Figure 9, will be approximately equal to the voltage drop across cathode resistor 98. This is because the grid to cathode voltage of the vacuum tube 24 will be small as explained above.

The circuit of Figure 11 is an improvement over the circuit of Figure 9 inasmuch as the transducer charging time is considerably reduced in the circuit of Figure 11 as compared to the charging time of the circuit of Figure 9. The polarizing potential developed in either circuit will be substantially less in magnitude than the potential of B-{.

A consideration of Equation 1 will show that it is desirable to have a polarizing potential as large as possible. A circuit which will increase the polarizing voltage to approximately B+ potential is shown in Figure 12 which schematically illustrates the microphone shown in Figure 8 having the diaphragm 16 and the back plate electrode 22. The back plate electrode is connected to the of a vacuum tube 166. The'plate168 of the tube 166 is connected to the source of operating voltage indicated at B+ through aplate current limiting resistor 170. The primary winding 172 of an output transformer174 is connected between the B-- terminal of the sourceof operating voltage and the cathode 176 of the tube. 166. The B terminal may be grounded, ifde'sired. The filament 178 of the tube 166 may be heated by direct current voltages in the manner shown inFigure 11. A capacitor 180 is connected between the terminal and the plate 16 so as to shunt any transients,- occurring in the power supply around the tube and winding 172 of the output transformer. In other words, theresistor170 and capacitor 180 function inthe same manner as the last section of the filter circuit 150 shown in Figure 10.

s A high polarizing voltage is immediately; established betweenthe diaphragm 16 and the back plate electrode 22 wheu the operating voltages are applied to the. circuit. Theip'er'ation ofthe circuit in quickly charging the electrostatic transducer to operating voltage is similar to that described in Figure 11. The polarizing voltage developed across the transducer elements of this circuit is greater than was the case with the circuit shown in Figure 11. When the circuit reaches its quiescent operating condition, the grid current conduction ceases and a bias voltage develops between the grid 164 and the cathode 176 which is sufficient to cause grid current cut off. The polarizing voltage across the transducer elements is equal to the voltage of the source of operating Voltage at B+, plus the bias voltage which is developed between the grid 164 and the cathode 176, since there' is substantially no direct-current voltage drop across the primary winding 172 of the output transformer 174. The bias voltage between the grid 164 and the cathode 176 is only a few volts. Therefore, the polarizing voltage developed across the transducer will be approximately equal to the voltage of the source of operating voltage at B+. This higher polarizing voltage results in a higher output voltage from the microphone.

Figure 13 shows another embodiment of the improved circuit for operating an electrostatic microphone. This circuit is particularly advantageous when the source of operating voltage for the microphone is a battery or batteries. The microphone elements are shown schematically as including the diaphragm 16, back plate electrode 22 and case The diaphragm 16 is connected over lead 114 to the positive terminal of a battery 182 which provides the source of high operating voltage for the circuit and microphone. The manner of connection of the microphone elements and the disposition of the lead 114 is illustrated in Figure 8 of the drawings. The back plate electrode 22 is connected over the lead 60 to the grid 184 of a tube 186. The plate 188 of the tube 186 is connected to the battery 182 by way of a filter circuit including the current limiting resistor 190 and the shunt capacitor 192. This resistance-capacitance circuit operates in the same manner as the circuit of resistor 170 and capacitor 180 shown in Figure 12. The filament 194 functions as the cathode of the tube. It is a feature of this circuit that the filament is connected to the battery 1% which supplies the filament heating power through the primary windings 198 and 200 of an output transformer 202. The signals from the circuit are obtained across the secondary winding 204 of die transformer 202. The primary windings 198 and 200 are connected, in bucking relationship, as indicated by the dots at the ends thereof which are shown on the drawing, in series with the battery 196 and the filament 194. Thus, the filament current does not produce flux in the core of the transformer. The windings 198 and 200 are in parallel with the plate circuit of the tube. Thus, signals passing through the tube and the windings 198 and 200 in its cathode circuit will be reflected in the output voltage across the secondary winding 204. If desired,

14 the windings 198 and 209 may be wound in a bifilai' manner. The polarizing voltage is established in the electrostatic transducer in a rapid manner as was the case for the circuit illustrated in Figure 12. Similarly, the voltage between the diaphragm 16 and the back plate electrode 22 will be a higher polarizing voltage than possible with prior circuits as was explained in connection with Figure 12.

Another circuit incorporating improvements in addition to the improvements provided by the circuits shown in Figures 11 to 13 is illustrated in Figure 14. This circuit is similar in most respects to the circuit shown in Figure 12 except that (1) connections are made to ground instead of to B- and (2) a resistor 205 is connected between the grid 164 and ground.

In certain applications of an electrostatic microphone, for example in motion picture studio work, the microphone may be exposed to explosive sound,- such as gunshot sounds. After exposure to severe gunshot sounds the. microphone may su'ifer a momentary decrease of sensitivity. Such decrease may possibly be attributable to the effect of the gunshot sound on the capacitive transduce'i' element of the microphone which causes the grid 164 of the tube 166- to become positive. Upon becomirigpositive, the grid draws current. The capacitive transdueer element charges so that the back plate 22 becomes vsuflic'iently negative to reduce signal transmission thiough are tube 166 for a short interval of time.

The provision of the grid resistor 205 permits the transducer element to discharge and return to its normal charge immediately. after the gunshot sound terminates. Thus, the interval of decreased microphone sensitivity is alleviated. In all other respects the circuit of Figure 14 operates similarly with the circuit of Figure 12.

What is claimed is:

1. A circuit for operating an electrostatic transducer including two electrodes having opposed surfaces spaced from each other to define a gap therebetween comprising an electron control device having an input terminal and two output terminals, said input terminal being electrically isolated from said output terminals, means for applying operating voltage simultaneously to one of said electrodes and one of said output terminals for immediately energizing said device and said transducer, means for connecting the other of said electrodes to said input terminal, and means for deriving a signal output from said device connected between said output terminals.

2. A circuit for operating an electrostatic transducer including two electrodes having opposed surfaces spaced from each other to define a capacitive element comprising an electron control device having a control electrode, said control electrode being electrically free of said discharge and collecting electrodes, an electron discharge electrode and an electron collecting electrode, an output circuit, said source of operating voltage being connected between said collecting electrode and said discharge electrode through said output circuit, means for connecting one of said transducer electrodes to said source of operating voltage, and means for connecting said control electrode to the other of said transducer electrodes.

3. A circuit for operating an electrostatic transducer including two electrodes having opposed surfaces spaced from each other to define a gap therebetween, comprising an electron discharge device having a control electrode, an anode electrode and a cathode electrode, said control electrode being electrically free of said discharge and collecting electrodes, means for connecting said control electrode to one of said transducer electrodes, means for conecting said source of operating voltage simultaneously to said anode electrode and the other of said transducer electrodes for simultaneously energizing said transducer and said device, and means for deriving output voltages I connected between said cathode and said anode electrodes.

said resistor between said control electrode and said cathode.

5. A circuit for operating an electrostatic transducer having a pair of electrodes, said electrodes having opposed surfaces spaced from each other to define a capacitive element, an electron tube having a plate, a grid and a cathode, said grid being connected to one of said electrodes, an impedance element connected to said cathode having direct-current transmission characteristics, means for connecting a source of operating voltage between said plate and said element for energizing said tube, means for connecting said source to the other of said electrodes for immediately establishing a polarizing voltage across said electrodes simultaneously with energization of said tube, said grid being electrically isolated from said plate and cathode to provide a free grid, and means for deriving signal output voltages across said element.

6. A circuit for operating an electrostatic transducer including two electrodes having opposed surfaces spaced from each other to define a capacitive element comprising an electron tube having a grid, a plate and a cathode,

an output transformer having a primary and secondary Winding, a current limiting resistor, one of said electrodes being connected to said grid, said grid being otherwise a free grid, and means for connecting a source of 25 operating voltage simultaneously to said resistor and to the other of said electrodes for simultaneously energizing 1% said tube and establishing a polarizing voltage across said transducer electrodes, the output voltage from said circuit being derived from said secondary winding.

7. A circuit in accordance with claim 6 including a resistor connected between said grid and said primary winding.

8. A circuit for operating an electrostatic transducer having a pair of electrodes, said electrodes having opposed surfaces spaced from each other to define a capacitive element, an electron tube having a plate, a grid and a filament, an output transformer having a primary winding and a secondary winding, said primary winding having two sections, a filament supply voltage source, a plate supply voltage source, means for connecting said filament supply source in a series circuit with said filament and said primary winding sections, said primary winding sections being connected in bucking relationship with each other, one of said electrodes being connected to said grid, said grid being otherwise a free grid, and means for connecting said plate supply source simultaneously to said plate and to the other of said electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,678,967 Grosskopf May 18, 1954 

